Concrete sets as cement hydrates. Hydration is an exothermic reaction, which means that it generates heat, and that reactions proceed faster when the concrete is hot. Quick reacting cements, like CEM I 52.5 R, produce comparatively quickly high temperatures, but as many reactions happen within a short time, the heat production is high, too, and as a consequence of heat flow limitation due to thermal resistance of concrete and the high ambient temperature, the heat accumulates in the concrete—and the maximum temperature may exceed 60° C. This may lead to strong deformations and therefore induce cracking, as well as ettringite formation afterwards.
Also, when cement hydrates, it takes up water and grows crystals around the aggregate particles. In hot ambient conditions and when the hydration reaction proceeds rapidly, the crystals grow quickly but don't have time to grow strong. Early strength of the resulting concrete will be higher, but 28-day strength will suffer. If the resulting concrete is about 10° C. hotter than normal (for example, 31° C. instead of 21° C.), the ultimate compressive strength will be about 10% lower.
Furthermore, in hot ambient conditions, as the cement sets, slump decreases rapidly and more mixing water is needed. This can also contribute to lower strengths (as much as another 10%), and in integrally coloured concrete, can lead to variations in water content which can result in significant differences in concrete colour between adjacent pours.
Another potential problem in hot ambient conditions is surface drying or plastic shrinkage, which occurs as fresh concrete loses its moisture too quickly after placement but before any strength development has occurred. This type of shrinkage is affected by environmental effects of temperature (of the concrete and ambient), wind and relative humidity. It is a particular problem in hot ambient concreting.
Hence, it is vital to maintain the temperature of concrete at a satisfactory level during curing, which presents considerable challenges because of the wide ambient temperatures encountered when casting concrete. The ASCC (American Society for Concrete Contractors) recommends that the temperature of the concrete is maintained between 10 and 21° C., in particular below 21° C.
The problem has been addressed in several ways, such as the use of slow reacting cements, such as fly ash-based cements, the use of cool or chilled water, the use of cooled aggregates, adding ice to the concrete mixture, the use of apparatus for cooling concrete during curing (see US 2005/0223717, Bourgault and Dancey, 2005) and the use of retarders or retarders admixtures. Retarders provide that the heat release is distributed over a longer time. Retarders can be added at the plant or on the job site to delay concrete setting time, which can be very quick when the concrete is hot. Retarders provide extra time but they also give the concrete more time to dry out, so curing is critical. Usually, the use of retarders also comes at the price of delayed setting. In case of hot ambient temperatures, for instance in the Middle East region, hydration delay and aimed construction progress need to be balanced, and this may present a challenge. In case of pre-cast concrete production, the heat issue leads to tension problems, too. And the risk of overheating by steam induced heat push is given as well. Also, if too much retarder is added to the concrete, used for a slab, it can lead to crusting, where the surface sets but the concrete below is still soft. This can reduce the flatness and even lead to delamination of the surface.
It is known to use 2 weight % of calcium nitrate as a setting accelerator in an admixture up to a temperature of 23° C., where setting is shifted to an earlier stage in the curing process (Influence of setting accelerators on chemical shrinkage of portland cement, Clemmens et al. American Concrete Institute, SP (2001), SP-200-15 (Fifth CANMET/ACI International Conference on Recent Advances in Concrete Technology, 2001), p. 235-249).
It is also known that calcium nitrate is able to shift the maximum heat release to an earlier point of time in the curing process. By this, the hydration energy is distributed over a period of time where without it, just low activity would happen. Hence, the setting starts very early, but heat is released more evenly (Hardening retarders for massive concrete, Justnes et al. American Concrete Institute, SP (2008), SP-253, p. 41-56 using maximum 1.5 weight % of calcium nitrate as setting accelerator in combination with a minor amount (0.1-0.3%) of a strong setting retardant like urea and organic acids (such as citric acid and tartaric acid). It was demonstrated (see FIG. 3) that, at a temperature of 40° C., the rate of heat evolution for a reference cement paste (without calcium nitrate) and for a paste comprising 1.5 weight % of calcium nitrate, is similar, and that the admixture combination (calcium nitrate and setting retardant) at 40° C. may not function in the practical semi-adiabatic case of massive concrete.
Hence, there is a need for a cementitious composition that can be produced and cured at hot ambient temperatures and wherein the heat release is distributed over time, and wherein the maximum temperature is lowered upon curing.